A 15-year-old boy has been kicked in the head and robbed in a nasty bus assault, before the offenders also allegedly attempted to extort cash from his mother.

The boy boarded a bus on Plenty Road in Bundoora at about 5pm on Tuesday and was sitting in his seat when he was kicked in the back of the head without warning.

A gang of three youths allegedly made threats and demanded the boy hand over his phone and watch.

They then got off the bus at South Morang with the 15-year-old’s belongings, and contacted his mother using his phone to make demands for cash.

Police found three teenagers at Lalor Train Station overnight and arrested them in relation to the robbery.

Two of them — an 18-year-old from Moonee Ponds and a 15-year-old from Tarneit — have been charged and released on bail.

A third teenager, a 16-year-old male from Werribee, required hospital treatment and is assisting police. He’s expected to be charged later today.

Press PLAY below to hear more details from police